Medical monitoring devices provide crucial data regarding a patient's medical condition. Two main types of monitors can be distinguished: those that monitor parameters that are a direct measure of one of the patient's physiological functions and those that monitor parameters that are an indirect measure of the status of a physiological function. An example of a parameter that is a direct measure of a physiological function is capnography, which measures and provides values of the CO2 concentration in the ventilated breath, a direct measure of the patient's ventilation functioning. An example of a parameter that is an indirect measurement is blood pressure, which indirectly provides information regarding the functioning of the heart. In addition, some of the monitored parameters crucial for evaluating the patient's condition may be “rates” (for example, heart rate, breath rate) while others may be “snap shots”, instantaneous type parameters (for example blood pressure, CO2, O2 concentrations).
Capnography is a non-invasive monitoring method used to continuously measure CO2 concentration in exhaled breath. CO2 is a constant metabolism product of the cells, and it is transported through the blood system to the lungs, from which it is eliminated through the alveolar membrane. The CO2 is exhaled out of the body and the concentration of the exhaled CO2, also known as end tidal CO2 (EtCO2) is an approximate estimation of the alveolar CO2 pressure and thus of the arterial levels of CO2. The measurements of the CO2 concentration in a breath cycle are performed by a capnographer, and the results are a numerical value displayed also in a graphical format in the shape of a waveform named a capnogram. The numerical value of the results may be presented in units of pressure (mm Hg) or as a percentile. The capnogram may depict CO2 concentration against total expired volume, but the more common capnogram illustrates CO2 concentration against time.
Capnography is widely used today as an important tool for tracking a patient's ventilation status in various health care settings, such as in an Emergency Room (ER), Operation Room (OR), Intensive Care Unit (ICU) and Emergency Medical Services (EMS). Among the clinical applications in which capnography may be used are Cardiovascular (for example in CPR, shock, pulmonary embolism), Respiratory (for example, verification of endotracheal tubing, mechanically ventilated patients, conditions such as Asthma, hyperventilation, hypoventilation, apnea; Sedation (for example during operation); Patient transport (both intra- and inter-hospital), and the like.
Analyzing the capnogram may yield valuable information about the patient's clinical status. A normal capnogram exhibits one or more typical waveforms, each one represents a single respiratory cycle, and deviations from the normal waveform may hint as to the clinical situation of the patient. For example, an abnormally high basal line represents re-breathing of exhaled CO2; a slow increase in CO2 concentration may suggest uneven emptying of the lungs; a rise in CO2 concentration without reaching a plateau may hint as to situations of asthma or other lower airway obstruction, very small changes in CO2 concentration may indicate an apnea situation, and so forth.
In addition to displaying respiratory cycles, a trend display is also available in which many individual consecutive breath cycles are compressed together so that changes over time may be easily distinguished, providing yet an additional aid in assessing and monitoring the patient's ventilation and clinical profile.
An additional breathing related parameter useful in monitoring a patient's clinical status is the respiratory rate. Respiratory rate is defined as the number of breaths taken in a minute, and it may change under various physiological and medical conditions. The rate may be abnormally high (tachypnea), abnormally low (bradypnea) or non-existent (apnea).
To further assist the health care providers in monitoring the patient's clinical status, the medical monitoring device may set off an alert when deviations from the standard are detected. For example, a capnographer may set off an alert (an audio and or visible alarm) when deviations or changes in the patient's capnogram are detected, either in a single breathing cycle, or in the trend display. Likewise, when the respiratory rate deviates from normal, the monitor may set off an alarm, alerting the health care providers as to the change.
However, the assessment of issuing an alert by a medical monitoring device is complicated and includes many considerations for determining and balancing various parameters such as threshold limits, measurement time and rates in order to minimize false, non-clinically significant alerts (that result from artifacts) over genuine alerts (that result from authentic physiological change). Therefore, the need arises to develop such methods to improve the meaningfulness of the estimation of the measured parameter and hence the decision of issuing a clinically significant alert.